1. Field of the Invention
The invention relates to a method and system for determining the potential friction between a tyre for vehicles and a rolling surface.
2. Description of the Related Art
The incorporation of electronic devices inside the tyres is becoming increasingly important in order to increase the vehicle safety. Such devices may, for example, include sensors and other components suitable for obtaining information about various quantities of a tyre such as, temperature, pressure, acceleration, number of tyre revolutions, vehicle speed. Such devices may further include a transmitter (typically wireless) for sending out of the tyre (typically to a control unit on board of the vehicle) the information obtained by the sensors and a microprocessor suitable for gathering and processing signals coming from the sensors, prior to the transmission thereof. Optionally, such devices may also include a receiver (typically wireless) for receiving any information from the outside (for example, from a control unit on board of the vehicle).
In this context, analysing and monitoring the interaction between the tyres of a vehicle and the rolling surface whereon the same operate during the use thereof may provide useful information to the driver of a vehicle as a driving aid and/or provide useful information to automatic control systems the same vehicle may be provided with and/or provide useful information to other vehicles or infrastructures other than the vehicle itself in order to enable, for example, traffic control and monitoring systems. For example, such information may be used by a traffic control and monitoring system for providing warning signals and/or adjusting the speed limits in adaptive speed limit systems.
In particular, the estimate of the potential friction may be important for allowing the development of technologies for active control systems of a vehicle dynamics or for driver assistance systems (Advanced Driver Assistance Systems, or ADAS).
The potential friction is defined as the ordinate of the absolute maximum point in a kinetic friction/slip curve. The kinetic friction is defined as the ratio between the force exchanged in the plane of contact between the tyre and the rolling surface and the vertical load acting on the tyre.
FIG. 1 shows two exemplary patterns of the kinetic friction coefficient μk as a function of the slip σ for two different conditions of tyre-rolling surface system.
When the conditions of the tyre-rolling surface system vary, that is, the operating conditions of the tyre (such as vertical load acting on the tyre, tyre inflation pressure, speed, wear, temperature, etc.), the features of the tyre itself (structure, mixture, etc.), and/or the features and conditions of the rolling surface (presence of slippery elements such as snow, ice, leaves, roughness, etc.), the relationship between slip and kinetic friction is described by a different curve (μpot1, μpot2) and the potential friction μpot is correspondingly different.
In the practice, the potential friction fixes a limit condition beyond which the tyre grip conditions begin to degrade up to reaching an asymptotic condition in which, even with increasing slip, the friction coefficient remains substantially constant and lower than the maximum friction (i.e. than the potential friction itself).
Systems capable of estimating the potential friction in a tyre-rolling surface system are known in the art such as, for example, the same Bracking Antilock Systems (ABS). Such systems, however, determine the potential friction in braking conditions, when the forces exchanged at the tyre-rolling surface interface are close to the maximum value, that is, at high slips.
The Applicant is instead directing its development activities towards systems capable of determining the potential friction in a tyre-rolling surface system under conditions of “free rolling” or “steady state” of the system, that is, under stationary rolling conditions (for example, at a constant speed, substantially in the absence of braking and steering).
However, considering that the condition of “free rolling” or “steady state” corresponds to small slip values (e.g. less than 1%) and that the friction/slip curves relating to different conditions of the tyre-rolling surface system (which correspond to different potential frictions) are concentrated in the proximity of the origin, almost overlapped (see the region highlighted in FIG. 1), the determination of the potential friction in conditions of “free rolling” is extremely difficult.
The international patent application no. WO 2010/073272 in the name of the same Applicant describes a technique for determining the potential friction without the need of achieving high slips.
Bert Breuer et al. (“Methods and Instruments for On-Board Measurement of Tyre/Road Friction”, SAE Conference, Dearborn. USA 1994) and U. Eichhorn et al. (“Prediction and monitoring of tyre/road friction”, in Proc. FISITA 24th Congress: Safety, the Vehicle and the Road Tech. Paper, London, UK, 1992, vol. 2, pp. 67-74) describe a technique for determining the friction between a tyre and the road from the deformation of elements of the tyre tread.
In the development of systems capable of determining the potential friction in a tyre-rolling surface system under conditions of “free rolling” or “steady state” of the system, the Applicant has carried out activities of acquisition and analysis of curves related to the longitudinal and lateral acceleration component measured, revolution after revolution, by an accelerometer placed on the inner liner of a tyre. This based on the observation that friction is a force that opposes the relative motion between tyre and rolling surface along the longitudinal and lateral direction of the tyre.